The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
The present invention relates to the nondestructive testing or evaluation of physical properties of materials, more particularly to methods and apparatuses for accomplishing near-field inspection of materials, such as involving utilization of microwave radiation in association with materials such as metallic or composite materials.
Various kinds of near-field microwave inspection have been conducted with respect to various kinds of structures (e.g., composite or metallic structures) having an extended surface area. Generally, a near-field probe (for example, a open-ended rectangular waveguide probe or an open-ended coaxial probe) is used in conventional practice of near-field microwave inspection. Typically, the microwave inspection inherently incorporates or assumes a xe2x80x9cstandoff distancexe2x80x9d or xe2x80x9cliftoffxe2x80x9d of the near-field probe in relation to the surface area of the material being inspected.
The measurement results are usually sensitive to the changes in this standoff distance. Sometimes a change in the standoff distance is related to variation in surface roughness (or, synonymously expressed, surface height). For instance, in the case of glass reinforced epoxy composites, the change in the standoff distance can be caused by surface roughness/height variations in the composite skin.
It is generally important to distinguish between or among various types of defects. For instance, in the case of a composite laminate, it may be desirable that an internal defect such as a layer-layer disbond be distinguished from a defect on the surface such as related to impact damage. In order to differentiate between or among internal and external defects, the influence of standoff distance variation must somehow be accounted for.
In view of the foregoing, it is an object of the present invention to provide method and apparatus for effectuating near-field microwave nondestructive testing of an object in such a way as to more capably distinguish between internal physical characteristics and external physical characteristics.
The present invention features the neutralization of the effect of surface variation of the object in the context of near-field sensing. Provided by the present invention is a circuit which accounts for standoff distance variation and eliminates its influence from the final inspection system output. The inventive compensatory and equalizing circuit has been designed and successfully tested by the U.S. Navy and Colorado State University in association with open-ended rectangular waveguide probes.
According to typical embodiments of the present invention, the inventive apparatus is used in association with a sensing device which is capable of producing a nonconstant device signal for inspecting an object. The sensing device""s nonconstant device signal varies in accordance with the distance of the sensing device from the object. The inventive apparatus comprises means for rendering the sensing device capable of producing a constant device signal at least until reaching said object, wherein the constant device signal is constant regardless of the distance.
According to many inventive embodiments, the inventive means for rendering includes: means for producing a nonconstant counteractive output signal, the nonconstant counteractive signal varying in accordance with the distance; means for modifying the nonconstant device signal so as to become a nonconstant modified device signal which is commensurate with the nonconstant counteractive signal; and, means for combining signals, the means for combining signals including means for combining the nonconstant counteractive signal and the nonconstant modified device signal. The constant device signal is based on the combining of the nonconstant counteractive signal and the nonconstant modified device signal.
According to frequently preferred inventive practice, the inventive means for rendering also includes means for producing a constant offset signal. Thus, the means for combining signals includes means for combining the nonconstant counteractive signal, the nonconstant modified device signal and the constant offset signal. The constant device signal is based on the combining of the nonconstant modified device signal, the nonconstant counteractive signal and the constant offset signal.
In typical inventive practice, the nonconstant device signal, the nonconstant modified device signal and the nonconstant counteractive each vary linearly according to distance. Frequent inventive practice prescribes such linear variation in terms of voltage value. The xe2x80x9cconstancyxe2x80x9d characteristic of both the constant device signal and the the constant offset signal presupposes a nonvarying linearity of each constant signal, freqently manifested in inventive practice as a constancy (i.e., single-valued linearity or invariability) in voltage value. In contrast, the xe2x80x9cnonconstancyxe2x80x9d characteristic of the nonconstant device signal, the nonconstant modified device signal and the nonconstant counteractive signal entails a varying linearity of each nonconstant signal, frequently manifested in inventive practice as a nonconstancy (i.e., plural-valued linearity or linear variability) in voltage value.
Usually, the nonconstant device signal varies linearly in accordance with the standoff distance; however, the inventive principles are still applicable whether the nonconstant device signal varies linearly or nonlinearly in accordance with the standoff distance. In fact, the present invention can be practiced regardless of whether the nonconstant device signal, the nonconstant modified device signal and the nonconstant counteractive signal vary linearly or nonlinearly according to distance. If, for instance, the initial voltage output varies as a nonlinear function of standoff distance, according to this invention a counterbalancing voltage output can be effected which equally but oppositely varies as a nonlinear function of standoff distance. Similarly, if the initial voltage output varies as a linear function of standoff distance, according to this invention a counterbalancing voltage output can be effected which equally but oppositely varies as a linear function of standoff distance.
Featured by the present invention is the provision of a voltage commensurate with the inspected material""s surface roughness, and the addition of such provided voltage to, or the subtraction of such provided voltage from, the voltage detected by the microwave detector. In other words, according to this invention, a voltage is provided which is proportional to the surface roughness and is then added to or subtracted from the voltage detected by the microwave detector; such proportionality of voltage with respect to surface roughness can equivalently be considered to be a proportionality of voltage with respect to standoff distance. In this way, the present invention renders the final output voltage independent of surface roughness variations, which are typically slight but which can manifest diverse degrees and kinds of irregularity.
A near-field microwave device typically produces a voltage output signal which is a linear function of standoff distance. According to the present invention, potentiometer circuitry is provided to produce a voltage output signal which is a linear function of standoff distance, but which is oppositely sloped in comparison with the voltage output signal of the microwave device. Thus, if the microwave device""s voltage output linearly increases in accordance with standoff distance, the inventive potentiometer circuitry""s voltage output linearly decreases in accordance with standoff distance; on the other hand, if the microwave device""s voltage output linearly decreases in accordance with standoff distance, the inventive potentiometer circuitry""s voltage output linearly increases in accordance with standoff distance.
Further, according to the present invention, the slope of the microwave device""s voltage output is rendered not only opposite to but also equal in magnitude to that of the potentiometer circuitry""s voltage output. In this regard, the microwave device""s voltage output is multiplied by an appropriate multiplication factor, thereby yielding a slope which is not only oppositely signed but which also has a magnitude which is equal to that of the potentiometer circuitry""s output voltage.
Therefore, in accordance with this invention, when the microwave device""s voltage output is multiplicatively modified and then counteractively (e.g., additively or subtractively) associated with the potentiometer circuitry""s voltage output, the result is a constant voltage output irrespective of standoff distance. If graphically visualized as voltage output (y-axis) as a function of standoff distance (x-axis), the voltage output of the near-field microwave device, as inventively modified, is zero-sloped (i.e., horizontal).
The present invention thus enhances or improves the inspection capability of near-field microwave nondestructive testing techniques (such as those which implement open-ended rectangular waveguide sensors or open-ended coaxial sensors) for detection of interior flaws (e.g., manufactured or in-service produced flaws) in materials (such as multi-layered dielectric composites) in which a certain degree of surface roughness is present.
Although the present invention is applicable to diverse types of materials such as generally categorized as composite materials, it is especially beneficial when practiced with respect to composite laminates, wherein it is desirable to distinguish internal anomalies (such as associated with bonding of lamina) from external (surface) irregularities. Currently, there is no known efficient technique for continuously correcting for standoff distance variations caused by surface roughness in structures (such as dielectric composite structures) as a near-field probe (such as an open-ended rectangular waveguide aperture probe) scans over a material (such as a composite material). The new circuitry according to this invention monitors standoff distance variations (e.g., due to surface roughness) and electronically corrects the microwave detector output voltage for this variation.
Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.